Lithography is a process by which patterns are created on a chip on a semiconductor wafer. For the last several decades, optical projection lithography has been the lithographic technique used in the high-volume manufacturing of integrated circuits. Over time, feature sizes on chips are getting smaller that requires more and more sophisticated lithography technologies, which enable to print such small features. In order to print ever smaller features of the patterns defining integrated circuits onto semiconductor wafers, the wavelength of the light used to project image of a pattern onto the wafer is continuously reduced. Extreme Ultraviolet lithography (“EUVL”) is one of the lithography technologies, which employs short wavelength radiation (“light”) in the approximate range of 10 nanometers (“nm”) to 14 nm that enables to print features having a size smaller than 100 nm. Because extreme ultraviolet (“EUV”) radiation is absorbed in almost all materials, a mask used in the EUVL is a reflective mask. The reflective mask, to transfer a pattern onto the wafer, reflects the radiation in certain regions and absorbs the radiation in other regions of the mask. Typical EUVL reflective mask blank includes a mirror deposited on a substrate, wherein the mirror consists of alternating layers of silicon and molybdenum, to maximize reflectivity of the light. The mirror of the EUVL mask blank is coated with a layer of an absorbing material. The absorbing material is patterned in a specific way to produce a mask. Typical factors that contribute to pattern displacement errors in printing a pattern onto a wafer include non-flatness of a substrate of a mask, variation of the thickness of the substrate of the mask, deformation of the mask due to stress induced by various layers deposited on the substrate of the mask, for example, reflective layers, a buffer layer, and an absorber layer. The use of EUV light and reflectance to print the patterns impose strict requirements on the quality and flatness of the substrate and the mask to reduce such errors. Current EUVL standard (“SEMI P37”) for a substrate for the EUVL mask specifies that the substrate frontside and backside non-flatness be no more than 50 nm peak-to-valley (“p-v”).
FIGS. 1A to 1C illustrate various types of a non-flatness of a substrate of a mask. FIG. 1A is a cross-sectional view 100 of the substrate 101, wherein the non-flatness of the substrate 101 has a shape of a slope 103 gradually extending from one part 102 of the front surface to the other part 104 of the front surface of the substrate 101. FIG. 1B is a cross-sectional view 110 of the substrate 111 of the mask, wherein the non-flatness is a bump defect 113, which lifts a portion of the front surface 112 of the substrate 111 relative to the other portions of the front surface 112. FIG. 1C is a cross-sectional view 120 of the substrate 121 of the mask, wherein both the front surface 122 and a back surface 123 of the substrate 121 have non-flatness portions. Each of the front surface 122 and the back surface 123 has a bump defect 124 and a depression defect 125, as shown in FIG. 1C.
Present polishing techniques can reduce non-flatness of the substrate down to 100 nm p-v. To further reduce non-flatness of the substrate, magnetorheological finishing (“MRF”) and ion beam figuring (“IBF”) are used. Each of MRF and IBF methods, however, involves removing physically of a material of the substrate that destroys a surface of a substrate and increases the roughness of the surface of the substrate at least by a factor of 5. For example, the roughness of the surface of the substrate may be higher than the 0.15 nm rms value specified in SEMI-P37 standard, as a result of using these method. The increased roughness of the substrate increases scattering of the light from the mask, reduces the reflectivity of the mask, and results in errors in printing the pattern onto the wafer. Also, neither MRF, nor IBF removes the local defects having a size smaller than 50 nm from the substrate. In addition, both the MRF and the IBF methods require high precision surface non-flatness measuring equipment to perform the correction. For the MRF and the IBF methods, correcting a surface non-flatness is performed through series of iterations, wherein each iteration involves measuring the size of the non-flatness via high precision interferometer, physically removing a portion of the surface material, then measuring the non-flatness in the same location again. Typically, it requires many iterations to correct the non-flatness in a single location on the surface. Each of the MRF and IBF methods is time consuming that significantly reduces throughput for a mask manufacturing and is also very expensive.